JP2694368B2 - Method for producing silicon nitride based sintered body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a silicon nitride sintered body which is suitable for a heat engine such as a gas turbine or a turbo rotor and which has excellent bending strength and toughness at high temperatures. .

(Prior Art) Conventionally, silicon nitride-based sintered bodies have attracted attention as engineering ceramics, particularly as materials for heat engines, because of their excellent strength, hardness, and thermochemical stability at high temperatures.

Generally, in order to manufacture these silicon nitride sintered bodies, since silicon nitride itself is difficult to sinter, various sintering aids such as rare earth element oxides are added, and hot pressing method and atmospheric pressure firing method are added. The gas pressure firing method is adopted. In addition, recently, a method of forming an impermeable seal made of glass or the like on the surface of a silicon nitride molded body having a desired composition and firing it under high pressure, assuming that a sintered body having high density and high strength can be obtained (Hereinafter referred to as "Seal HIP") is being studied.

On the other hand, from the viewpoint of composition, as described above, in addition to rare earth element oxides such as Y ₂ O _3, oxides such as Al ₂ O ₃ and MgO are most commonly used as sintering aids. In consideration of the high temperature characteristics of the sintered body, when Al ₂ O ₃ and MgO are included, a low melting point substance is generated at the grain boundary of the sintered body, and the high temperature strength and high temperature oxidation resistance are reduced. Therefore, a composition consisting of a simple ternary system of Si ₃ N ₄ -RE ₂ O ₃ (rare earth oxide) -SiO ₂ which is substantially free of the above oxides has been investigated.

Further, from the viewpoint of the structure of the sintered body, the grain boundary phase in the sintered body has been attracting attention as a factor that determines the high temperature characteristics, and the grain boundary phase is changed to improve the strength of the grain boundary phase itself. Attempts have been made to substantially crystallize. Therefore, recently, for the above-mentioned simple ternary composition, Si ₃ N ₄ -RE ₂ O _{3 was} added to the grain boundaries by studying the firing conditions or heat treatment of the sintered body.
Various crystal phases composed of (rare earth oxide) -SiO ₂ such as melilite, apatite, YAM, wollastonite, etc. are also deposited.

(Problems to be Solved by the Invention) However, although crystallization of the grain boundary phase has some effect on the high temperature strength, it is very difficult to stably precipitate only a specific crystal phase at the grain boundary. In some cases, the characteristics may be deteriorated by the formation of a glass phase having a low melting point in the grain boundary phase in addition to the crystal phase.

Therefore, the present inventors have considered that the above ternary composition
The composition having a SiO ₂ / RE ₂ O ₃ molar ratio of more than ₂ and an excess of 25 or less of SiO ₂ is fired by the seal HIP method to have a fine granular structure, and the grain boundary phase is composed of silicon, a rare earth element, oxygen, It was proposed that a sintered body composed of nitrogen, which has excellent room temperature strength, high temperature strength, and oxidation resistance, can be obtained.

The above-mentioned sintered body has excellent properties in bending strength and oxidation resistance as compared with conventional products, but due to the fine granular structure, with respect to the development of cracks from the outside,
It has been found that there is a drawback that the toughness of the so-called sintered body itself is apt to be deteriorated because it does not have sufficient effect.

(Object of the Invention) It is an object of the present invention to provide a method for producing a sintered body having excellent toughness while maintaining the above-mentioned excellent strength and oxidation resistance.

(Means for Solving Problems) As a result of studying the above problems, the inventors of the present invention have found that a high-density silicon nitride dense body with a specific ratio containing a large amount of α-type silicon nitride has a high temperature. The silicon nitride crystal grains are needle-shaped to form a fibrous structure by the heat treatment at, which allows the sintered body to have high toughness (K _1c ) while maintaining room temperature bending strength and high temperature bending strength. It was found that it can be given.

That is, according to the present invention, 70 to 99 mol% of silicon nitride, 0.1 to 5 mol% of rare earth element oxide, and excess oxygen (SiO ₂ conversion amount) 2
5 mol% or less, the (excess oxygen / rare earth element) molar ratio is greater than 2 and within the range of 25 or less.
Provided is a method for producing a silicon nitride sintered body, which comprises treating a silicon nitride sintered body having a conversion rate of 30% or more and a theoretical density ratio of 95% or more in a non-oxidizing atmosphere at 1500 to 1800 ° C. It

 Hereinafter, the present invention will be described in detail.

According to the present invention, the composition is 70 to 99 mol% of silicon nitride, especially 80 to 93.5 mol%, and 0.1 to 5 mol% of rare earth element oxides, especially 0.5 to 4 mol%, and excess oxygen (SiO ₂ conversion amount). At 25
Molar ratio is less than or equal to 6% to 20%, and the molar ratio (excess oxygen / rare earth oxide) is greater than 2.
25 or less, especially 3 to 20 and the α of silicon nitride
Prepare a sintered body with a conversion rate of 30% or more and a theoretical density ratio of 95% or more. The excess oxygen is the total oxygen amount contained in the entire system of the sintered body, which is the oxygen amount excluding oxygen mixed in as a rare earth element oxide in a stoichiometric amount, specifically, impurities in the silicon nitride raw material. It is composed of oxygen or oxygen added as SiO ₂ , and in the present invention, all show the equivalent amount of SiO ₂ .

In the present invention, as a method for producing a silicon nitride-based sintered body having a high α-conversion rate, the present inventors have previously proposed Japanese Patent Application No. 63-33241.
According to the method proposed in No. 1, the glass seal HIP method can be adopted. Specifically, as raw material powder, silicon nitride powder, rare earth element oxide powder, and optionally SiO ₂
Use powder. Silicon nitride powder is used for BE because it promotes sinterability.
It is desirable that the T specific surface area is 3 to ²⁰ m ² / g and the α-conversion rate is 95% or more, and the oxygen content is generally 0.8 to 1.4 wt% as a commercial product, but it can be arbitrarily added by adding SiO _2. Can be adjusted.

These powders are weighed and mixed with the above-mentioned composition, a binder is added and granulated, and then molded. A well-known method can be adopted for the molding, and specifically, press molding, extrusion molding, casting molding, injection molding and the like can be adopted.

After debinding the molded body obtained in this manner, a glass such as BN powder and a powder having poor wettability are formed on the surface of the molded body in order to prevent the reaction with the glass or the like as the sealing material in the firing step. Apply. BN on surface of compact
For the coating of powder having poor wettability with glass, etc., powder of BN or the like may be made into a slurry and applied to the molded body, or the slurry may be spray-applied. The coating amount on the surface of the molded body is preferably about 1 to 10 mm.

Next, a glass powder that forms a seal at the time of firing is applied to the surface of the molded body to which BN has been applied, or the molded body is encapsulated in a glass capsule. As another method, the molded body can be buried in a heat-resistant container filled with glass powder. After that, the molded body
Baking at high temperature and high pressure by HIP method.

Firing is first at least the softening point of the glass present on the surface of the molded body, and at the same time the temperature is raised to the firing temperature, which is equal to or higher than the decomposition equilibrium pressure of silicon nitride at that temperature 0.01 to 0.2 M
A partial pressure nitrogen gas having a high Pa level is introduced to soften the glass and form a gas impermeable film of glass on the surface of the molded body. After the gas impermeable film is completely formed on the surface of the molded body, under the condition that the pressure in the furnace can be sufficiently densified, for example, 50
Increase the pressure to above MPa. An inert gas such as nitrogen or argon is used as the pressure medium at this time. At this stage, a liquid phase is generated by the rare earth oxide, SiO ₂ and silicon nitride, and firing progresses, and the densification of the liquid phase is almost completed. Thereafter, the temperature and the pressure are both reduced to end the firing. The firing temperature during this firing is set to 1450 to 1800 ° C, particularly 1500 to 1750 ° C.

According to such a method, since the α-β transition is unlikely to occur due to the low firing temperature, a large amount of α-type silicon nitride can be left, and the sintered body itself can be densified. it can.

A feature of the present invention is that the silicon nitride sintered body having the above-mentioned specific properties is heat-treated in a non-oxidizing atmosphere of 1500 to 1800 ° C., particularly 1550 to 1750 ° C., specifically, an atmosphere of nitrogen, argon or the like. .

By such heat treatment, the α-type silicon nitride remaining in the sintered body undergoes α-β transition, and at the same time, the acicularization of the silicon nitride crystal proceeds, and a fibrous structure of silicon nitride is formed in the sintered body. It In addition, since acicularization during processing progresses in a highly dense material, compressive stress occurs between silicon nitride particles,
This makes it possible to significantly improve the toughness of the sintered body while maintaining the excellent strength and oxidation resistance before the treatment.

The thus obtained sintered body is a needle-shaped β-type silicon nitride crystal phase, specifically, a crystal phase having an average aspect ratio of 3 or more, and optionally a small amount of α-type silicon nitride crystal phase, Although silicon, rare earth elements, oxygen and nitrogen are present at the grain boundaries of the phase, according to the above-mentioned manufacturing method, the silicon oxynitride crystal phase represented by Si ₂ N ₂ O is usually used in addition to the high melting point glass phase. Alternatively, a disilicate crystal phase represented by RE ₂ O ₃ .2SiO ₂ may be formed. Such a grain boundary structure has advantages in that it has excellent properties in high-temperature strength and oxidation resistance as compared with conventionally known various crystal phases and can be manufactured stably.

In the present invention, the reason for limiting the molar ratio of excess oxygen and rare earth element oxide of the sintered body to be treated to the above range is all related to the characteristics of the final sintered body. If it is 2 or less, the oxidation resistance at high temperature tends to deteriorate, and conversely, if it exceeds 25, a glass having a low melting point is likely to be produced, and the high temperature characteristics deteriorate. Further, even if any of silicon nitride, rare earth oxide, and excess oxygen deviates from the above range, room temperature strength and high temperature strength deteriorate. Further, the α-conversion rate of silicon nitride is limited to the above range, because when the α-conversion rate is less than 30%, the acicularization of silicon nitride due to α-β transition during heat treatment becomes insufficient, and If the toughness is not achieved and the theoretical density ratio is lower than 95%, the final sintered body is insufficiently densified and the strength at room temperature and high temperature is deteriorated.

As the rare earth element oxide used in the present invention, Y ₂ O ₃ is most common, but Yb ₂ O ₃ , Er ₂ O ₃ , Ho
Heavy rare earth element oxides such as ₂ O ₃ and Dy ₂ O ₃ are preferable from the viewpoint of stability of characteristics.

 Hereinafter, the present invention will be described with reference to the following examples.

(Example) Silicon nitride powder (BET specific surface area 5 m ² /
g, α conversion rate 99%, impurity oxygen amount 1.0% by weight) and various rare earth oxides or SiO ₂ powders are mixed and mixed to have the composition shown in Table 1, and then pressed at 1 t / cm ² . 14 after molding
It was calcined at 00 ° C.

A BN powder paste having a particle size of 1 to 5 μm was applied to the obtained molded body in a thickness of 1 to 10 mm, and then glass containing SiO ₂ as a main component was applied in a thickness of 1 to 10 mm.

The compact thus treated was placed in a hot isostatic firing furnace and fired under various conditions to prepare 16 kinds of sintered bodies having different characteristics shown in Table 1.

Next, using these sintered bodies, heat treatment was performed under the conditions shown in Table 2.

For each sintered body before and after heat treatment, the four-point bending bending strength at room temperature and 1400 ° C. and the toughness value (K _1c ) were measured by the 1M method according to JIS R1601. Furthermore, the increase in the weight of oxidization at 1400 ° C was investigated. Further, the average aspect ratio and the average grain size (major axis) of the silicon nitride crystal grains were measured from a mirror surface photograph of the sintered body after the heat treatment.

 The results are shown in Table 2.

According to Table 1 and Table 2, Sample 6 in which the (excess oxygen / rare earth oxide) molar ratio of the sintered body used is less than 2
At room temperature and 1400 ° C., the strength was low, and in Sample 5, where the molar ratio exceeded 25, a decrease in high temperature strength was observed. Further, in Sample 7 in which the sintered body having the theoretical density ratio after HIP firing of less than 95% was heat-treated, the pores in the sintered body remained even after the heat treatment, and the strength decreased. Sample 8 obtained by heat-treating a sintered body with an alpha conversion rate of 30% or less
No needle-like structure was observed in the sample, and the effect of improving toughness was not obtained. In Sample 17, whose heat treatment temperature was lower than 1500 ° C, no change in the structure was observed, and in Sample 18, which was heat-treated at a temperature higher than 1800 ° C, abnormal grain growth was observed, and the strength was low.

In contrast to these comparative examples, all of the samples of the present invention have a remarkable acicular structure by heat treatment, which results in strength,
The toughness (K _1c ) could be improved without deterioration of the oxidation resistance. By the way, room temperature strength was 1000MPa or more, 1400 ℃ strength was 600MPa or more, and K _1c 6MPam ^1/2 or more was achieved.

(Effects of the Invention) As described above in detail, according to the silicon nitride sintered body of the present invention, the high-density silicon nitride sintered body of a specific composition containing a large amount of α-type silicon nitride is subjected to heat treatment. , The acicularization of silicon nitride due to α-β transition can be promoted, and a sintered body having a fibrous structure can be obtained, thereby significantly improving the toughness while maintaining the characteristics of the initial sintered body. You can

Therefore, the application of the silicon nitride sintered body as a material for various industrial structural parts such as a heat engine can be further expanded.

Claims (1)

(57) [Claims] 1. A method comprising 70 to 99 mol% of silicon nitride, 0.1 to 5 mol% of a rare earth element oxide, and 25 mol% or less of excess oxygen (equivalent to SiO ₂ ), and having a (excess oxygen / rare earth element) molar ratio. Two
1500 for a larger sintered body having a ratio of α of silicon nitride of 30% or more and a theoretical density ratio of 95% or more.
A method for producing a silicon nitride sintered body, which comprises treating at 1800C in a non-oxidizing atmosphere.